Add tests to verify assembler output -- Fix DoNotOptimize. (#530)

* Add tests to verify assembler output -- Fix DoNotOptimize.

For things like `DoNotOptimize`, `ClobberMemory`, and even `KeepRunning()`,
it is important exactly what assembly they generate. However, we currently
have no way to test this. Instead it must be manually validated every
time a change occurs -- including a change in compiler version.

This patch attempts to introduce a way to test the assembled output automatically.
It's mirrors how LLVM verifies compiler output, and it uses LLVM FileCheck to run
the tests in a similar way.

The tests function by generating the assembly for a test in CMake, and then
using FileCheck to verify the // CHECK lines in the source file are found
in the generated assembly.

Currently, the tests only run on 64-bit x86 systems under GCC and Clang,
and when FileCheck is found on the system.

Additionally, this patch tries to improve the code gen from DoNotOptimize.
This should probably be a separate change, but I needed something to test.

* Disable assembly tests on Bazel for now

* Link FIXME to github issue

* Fix Tests on OS X

* fix strip_asm.py to work on both Linux and OS X like targets

13 files changed, 734 insertions, 12 deletions

diff --git a/.gitignore b/.gitignore
index aca5f93..050e469 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,6 +6,7 @@
 *.dylib
 *.cmake
 !/cmake/*.cmake
+!/test/AssemblyTests.cmake
 *~
 *.pyc
 __pycache__
diff --git a/.travis.yml b/.travis.yml
index 137cc98..09c058c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -34,6 +34,7 @@ matrix:
       env:
         - INSTALL_GCC6_FROM_PPA=1
         - COMPILER=g++-6 C_COMPILER=gcc-6  BUILD_TYPE=Debug
+        - ENABLE_SANITIZER=1
         - EXTRA_FLAGS="-fno-omit-frame-pointer -g -O2 -fsanitize=undefined,address -fuse-ld=gold"
     - compiler: clang
       env: COMPILER=clang++ C_COMPILER=clang BUILD_TYPE=Debug
@@ -91,6 +92,7 @@ matrix:
       env:
         - COMPILER=clang++-3.8 C_COMPILER=clang-3.8 BUILD_TYPE=Debug
         - LIBCXX_BUILD=1 LIBCXX_SANITIZER="Undefined;Address"
+        - ENABLE_SANITIZER=1
         - EXTRA_FLAGS="-stdlib=libc++ -g -O2 -fno-omit-frame-pointer -fsanitize=undefined,address -fno-sanitize-recover=all"
         - UBSAN_OPTIONS=print_stacktrace=1
     # Clang w/ libc++ and MSAN
@@ -102,6 +104,7 @@ matrix:
       env:
         - COMPILER=clang++-3.8 C_COMPILER=clang-3.8 BUILD_TYPE=Debug
         - LIBCXX_BUILD=1 LIBCXX_SANITIZER=MemoryWithOrigins
+        - ENABLE_SANITIZER=1
         - EXTRA_FLAGS="-stdlib=libc++ -g -O2 -fno-omit-frame-pointer -fsanitize=memory -fsanitize-memory-track-origins"
     # Clang w/ libc++ and MSAN
     - compiler: clang
@@ -112,8 +115,8 @@ matrix:
       env:
         - COMPILER=clang++-3.8 C_COMPILER=clang-3.8 BUILD_TYPE=RelWithDebInfo
         - LIBCXX_BUILD=1 LIBCXX_SANITIZER=Thread
+        - ENABLE_SANITIZER=1
         - EXTRA_FLAGS="-stdlib=libc++ -g -O2 -fno-omit-frame-pointer -fsanitize=thread -fno-sanitize-recover=all"
-
     - os: osx
       osx_image: xcode8.3
       compiler: clang
@@ -131,15 +134,20 @@ matrix:
         - COMPILER=g++-7 C_COMPILER=gcc-7  BUILD_TYPE=Debug
 
 before_script:
-  - if [ -z "$BUILD_32_BITS" ]; then
-      export BUILD_32_BITS=OFF && echo disabling 32 bit build;
-    fi
   - if [ -n "${LIBCXX_BUILD}" ]; then
       source .travis-libcxx-setup.sh;
     fi
+  - if [ -n "${ENABLE_SANITIZER}" ]; then
+      export EXTRA_OPTIONS="-DBENCHMARK_ENABLE_ASSEMBLY_TESTS=OFF";
+    else
+      export EXTRA_OPTIONS="";
+    fi
   - mkdir -p build && cd build
 
 before_install:
+  - if [ -z "$BUILD_32_BITS" ]; then
+      export BUILD_32_BITS=OFF && echo disabling 32 bit build;
+    fi
   - if [ -n "${INSTALL_GCC6_FROM_PPA}" ]; then
       sudo add-apt-repository -y "ppa:ubuntu-toolchain-r/test";
       sudo apt-get update --option Acquire::Retries=100 --option Acquire::http::Timeout="60";
@@ -147,7 +155,11 @@ before_install:
 
 install:
   - if [ -n "${INSTALL_GCC6_FROM_PPA}" ]; then
-      sudo -E apt-get -yq --no-install-suggests --no-install-recommends --force-yes install g++-6;
+      sudo -E apt-get -yq --no-install-suggests --no-install-recommends install g++-6;
+    fi
+  - if [ "${TRAVIS_OS_NAME}" == "linux" -a "${BUILD_32_BITS}" == "OFF" ]; then
+      sudo -E apt-get -y --no-install-suggests --no-install-recommends install llvm-3.9-tools;
+      sudo cp /usr/lib/llvm-3.9/bin/FileCheck /usr/local/bin/;
     fi
   - if [ "${BUILD_TYPE}" == "Coverage" -a "${TRAVIS_OS_NAME}" == "linux" ]; then
       PATH=~/.local/bin:${PATH};
@@ -171,7 +183,7 @@ install:
     fi
 
 script:
-  - cmake -DCMAKE_C_COMPILER=${C_COMPILER} -DCMAKE_CXX_COMPILER=${COMPILER} -DCMAKE_BUILD_TYPE=${BUILD_TYPE} -DCMAKE_CXX_FLAGS="${EXTRA_FLAGS}" -DBENCHMARK_DOWNLOAD_DEPENDENCIES=ON -DBENCHMARK_BUILD_32_BITS=${BUILD_32_BITS} ..
+  - cmake -DCMAKE_C_COMPILER=${C_COMPILER} -DCMAKE_CXX_COMPILER=${COMPILER} -DCMAKE_BUILD_TYPE=${BUILD_TYPE} -DCMAKE_CXX_FLAGS="${EXTRA_FLAGS}" -DBENCHMARK_DOWNLOAD_DEPENDENCIES=ON -DBENCHMARK_BUILD_32_BITS=${BUILD_32_BITS} ${EXTRA_OPTIONS} ..
   - make
   - ctest -C ${BUILD_TYPE} --output-on-failure
   - bazel test -c dbg --define google_benchmark.have_regex=posix --announce_rc --verbose_failures --test_output=errors --keep_going //test/...
diff --git a/CMakeLists.txt b/CMakeLists.txt
index aa08267..4c10793 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -27,10 +27,48 @@ option(BENCHMARK_DOWNLOAD_DEPENDENCIES "Allow the downloading and in-tree buildi
 # in cases where it is not possible to build or find a valid version of gtest.
 option(BENCHMARK_ENABLE_GTEST_TESTS "Enable building the unit tests which depend on gtest" ON)
 
+set(ENABLE_ASSEMBLY_TESTS_DEFAULT OFF)
+function(should_enable_assembly_tests)
+  if(CMAKE_BUILD_TYPE)
+    string(TOLOWER ${CMAKE_BUILD_TYPE} CMAKE_BUILD_TYPE_LOWER)
+    if (${CMAKE_BUILD_TYPE_LOWER} MATCHES "coverage")
+      # FIXME: The --coverage flag needs to be removed when building assembly
+      # tests for this to work.
+      return()
+    endif()
+  endif()
+  if (MSVC)
+    return()
+  elseif(NOT CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64")
+    return()
+  elseif(NOT CMAKE_SIZEOF_VOID_P EQUAL 8)
+    # FIXME: Make these work on 32 bit builds
+    return()
+  elseif(BENCHMARK_BUILD_32_BITS)
+     # FIXME: Make these work on 32 bit builds
+    return()
+  endif()
+  find_program(LLVM_FILECHECK_EXE FileCheck)
+  if (LLVM_FILECHECK_EXE)
+    set(LLVM_FILECHECK_EXE "${LLVM_FILECHECK_EXE}" CACHE PATH "llvm filecheck" FORCE)
+    message(STATUS "LLVM FileCheck Found: ${LLVM_FILECHECK_EXE}")
+  else()
+    message(STATUS "Failed to find LLVM FileCheck")
+    return()
+  endif()
+  set(ENABLE_ASSEMBLY_TESTS_DEFAULT ON PARENT_SCOPE)
+endfunction()
+should_enable_assembly_tests()
+
+# This option disables the building and running of the assembly verification tests
+option(BENCHMARK_ENABLE_ASSEMBLY_TESTS "Enable building and running the assembly tests"
+    ${ENABLE_ASSEMBLY_TESTS_DEFAULT})
+
 # Make sure we can import out CMake functions
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules")
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 
+
 # Read the git tags to determine the project version
 include(GetGitVersion)
 get_git_version(GIT_VERSION)
diff --git a/README.md b/README.md
index 167bf7a..c8f8c01 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,8 @@ IRC channel: https://freenode.net #googlebenchmark
 
 [Additional Tooling Documentation](docs/tools.md)
 
+[Assembly Testing Documentation](docs/AssemblyTests.md)
+
 
 ## Building
 
diff --git a/docs/AssemblyTests.md b/docs/AssemblyTests.md
new file mode 100644
index 0000000..1fbdc26
--- /dev/null
+++ b/docs/AssemblyTests.md
@@ -0,0 +1,147 @@
+# Assembly Tests
+
+The Benchmark library provides a number of functions whose primary
+purpose in to affect assembly generation, including `DoNotOptimize`
+and `ClobberMemory`. In addition there are other functions,
+such as `KeepRunning`, for which generating good assembly is paramount.
+
+For these functions it's important to have tests that verify the
+correctness and quality of the implementation. This requires testing
+the code generated by the compiler.
+
+This document describes how the Benchmark library tests compiler output,
+as well as how to properly write new tests.
+
+
+## Anatomy of a Test
+
+Writing a test has two steps:
+
+* Write the code you want to generate assembly for.
+* Add `// CHECK` lines to match against the verified assembly.
+
+Example:
+```c++
+
+// CHECK-LABEL: test_add:
+extern "C" int test_add() {
+    extern int ExternInt;
+    return ExternInt + 1;
+
+    // CHECK: movl ExternInt(%rip), %eax
+    // CHECK: addl %eax
+    // CHECK: ret
+}
+
+```
+
+#### LLVM Filecheck
+
+[LLVM's Filecheck](https://llvm.org/docs/CommandGuide/FileCheck.html)
+is used to test the generated assembly against the `// CHECK` lines
+specified in the tests source file. Please see the documentation
+linked above for information on how to write `CHECK` directives.
+
+#### Tips and Tricks:
+
+* Tests should match the minimal amount of output required to establish
+correctness. `CHECK` directives don't have to match on the exact next line
+after the previous match, so tests should omit checks for unimportant
+bits of assembly. ([`CHECK-NEXT`](https://llvm.org/docs/CommandGuide/FileCheck.html#the-check-next-directive)
+can be used to ensure a match occurs exactly after the previous match).
+
+* The tests are compiled with `-O3 -g0`. So we're only testing the
+optimized output.
+
+* The assembly output is further cleaned up using `tools/strip_asm.py`.
+This removes comments, assembler directives, and unused labels before
+the test is run.
+
+* The generated and stripped assembly file for a test is output under
+`<build-directory>/test/<test-name>.s`
+
+* Filecheck supports using [`CHECK` prefixes](https://llvm.org/docs/CommandGuide/FileCheck.html#cmdoption-check-prefixes)
+to specify lines that should only match in certain situations.
+The Benchmark tests use `CHECK-CLANG` and `CHECK-GNU` for lines that
+are only expected to match Clang or GCC's output respectively. Normal
+`CHECK` lines match against all compilers. (Note: `CHECK-NOT` and
+`CHECK-LABEL` are NOT prefixes. They are versions of non-prefixed
+`CHECK` lines)
+
+* Use `extern "C"` to disable name mangling for specific functions. This
+makes them easier to name in the `CHECK` lines.
+
+
+## Problems Writing Portable Tests
+
+Writing tests which check the code generated by a compiler are
+inherently non-portable. Different compilers and even different compiler
+versions may generate entirely different code. The Benchmark tests
+must tolerate this.
+
+LLVM Filecheck provides a number of mechanisms to help write
+"more portable" tests; including [matching using regular expressions](https://llvm.org/docs/CommandGuide/FileCheck.html#filecheck-pattern-matching-syntax),
+allowing the creation of [named variables](https://llvm.org/docs/CommandGuide/FileCheck.html#filecheck-variables)
+for later matching, and [checking non-sequential matches](https://llvm.org/docs/CommandGuide/FileCheck.html#the-check-dag-directive).
+
+#### Capturing Variables
+
+For example, say GCC stores a variable in a register but Clang stores
+it in memory. To write a test that tolerates both cases we "capture"
+the destination of the store, and then use the captured expression
+to write the remainder of the test.
+
+```c++
+// CHECK-LABEL: test_div_no_op_into_shr:
+extern "C" void test_div_no_op_into_shr(int value) {
+    int divisor = 2;
+    benchmark::DoNotOptimize(divisor); // hide the value from the optimizer
+    return value / divisor;
+
+    // CHECK: movl $2, [[DEST:.*]]
+    // CHECK: idivl [[DEST]]
+    // CHECK: ret
+}
+```
+
+#### Using Regular Expressions to Match Differing Output
+
+Often tests require testing assembly lines which may subtly differ
+between compilers or compiler versions. A common example of this
+is matching stack frame addresses. In this case regular expressions
+can be used to match the differing bits of output. For example:
+
+```c++
+int ExternInt;
+struct Point { int x, y, z; };
+
+// CHECK-LABEL: test_store_point:
+extern "C" void test_store_point() {
+    Point p{ExternInt, ExternInt, ExternInt};
+    benchmark::DoNotOptimize(p);
+
+    // CHECK: movl ExternInt(%rip), %eax
+    // CHECK: movl %eax, -{{[0-9]+}}(%rsp)
+    // CHECK: movl %eax, -{{[0-9]+}}(%rsp)
+    // CHECK: movl %eax, -{{[0-9]+}}(%rsp)
+    // CHECK: ret
+}
+```
+
+## Current Requirements and Limitations
+
+The tests require Filecheck to be installed along the `PATH` of the
+build machine. Otherwise the tests will be disabled.
+
+Additionally, as mentioned in the previous section, codegen tests are
+inherently non-portable. Currently the tests are limited to:
+
+* x86_64 targets.
+* Compiled with GCC or Clang
+
+Further work could be done, at least on a limited basis, to extend the
+tests to other architectures and compilers (using `CHECK` prefixes).
+
+Furthermore, the tests fail for builds which specify additional flags
+that modify code generation, including `--coverage` or `-fsanitize=`.
+
diff --git a/include/benchmark/benchmark.h b/include/benchmark/benchmark.h
index 9fb1f55..04fbbf4 100644
--- a/include/benchmark/benchmark.h
+++ b/include/benchmark/benchmark.h
@@ -303,15 +303,20 @@ BENCHMARK_UNUSED static int stream_init_anchor = InitializeStreams();
 // See: https://youtu.be/nXaxk27zwlk?t=2441
 #ifndef BENCHMARK_HAS_NO_INLINE_ASSEMBLY
 template <class Tp>
-inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {
-  // Clang doesn't like the 'X' constraint on `value` and certain GCC versions
-  // don't like the 'g' constraint. Attempt to placate them both.
+inline BENCHMARK_ALWAYS_INLINE
+void DoNotOptimize(Tp const& value) {
+    asm volatile("" : : "r,m"(value) : "memory");
+}
+
+template <class Tp>
+inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp& value) {
 #if defined(__clang__)
-  asm volatile("" : : "g"(value) : "memory");
+  asm volatile("" : "+r,m"(value) : : "memory");
 #else
-  asm volatile("" : : "i,r,m"(value) : "memory");
+  asm volatile("" : "+m,r"(value) : : "memory");
 #endif
 }
+
 // Force the compiler to flush pending writes to global memory. Acts as an
 // effective read/write barrier
 inline BENCHMARK_ALWAYS_INLINE void ClobberMemory() {
diff --git a/test/AssemblyTests.cmake b/test/AssemblyTests.cmake
new file mode 100644
index 0000000..d8f321a
--- /dev/null
+++ b/test/AssemblyTests.cmake
@@ -0,0 +1,45 @@
+
+
+set(ASM_TEST_FLAGS "")
+check_cxx_compiler_flag(-O3 BENCHMARK_HAS_O3_FLAG)
+if (BENCHMARK_HAS_O3_FLAG)
+  list(APPEND ASM_TEST_FLAGS -O3)
+endif()
+
+check_cxx_compiler_flag(-g0 BENCHMARK_HAS_G0_FLAG)
+if (BENCHMARK_HAS_G0_FLAG)
+  list(APPEND ASM_TEST_FLAGS -g0)
+endif()
+
+check_cxx_compiler_flag(-fno-stack-protector BENCHMARK_HAS_FNO_STACK_PROTECTOR_FLAG)
+if (BENCHMARK_HAS_FNO_STACK_PROTECTOR_FLAG)
+  list(APPEND ASM_TEST_FLAGS -fno-stack-protector)
+endif()
+
+split_list(ASM_TEST_FLAGS)
+string(TOUPPER "${CMAKE_CXX_COMPILER_ID}" ASM_TEST_COMPILER)
+
+macro(add_filecheck_test name)
+  cmake_parse_arguments(ARG "" "" "CHECK_PREFIXES" ${ARGV})
+  add_library(${name} OBJECT ${name}.cc)
+  set_target_properties(${name} PROPERTIES COMPILE_FLAGS "-S ${ASM_TEST_FLAGS}")
+  set(ASM_OUTPUT_FILE "${CMAKE_CURRENT_BINARY_DIR}/${name}.s")
+  add_custom_target(copy_${name} ALL
+      COMMAND ${PROJECT_SOURCE_DIR}/tools/strip_asm.py
+        $<TARGET_OBJECTS:${name}>
+        ${ASM_OUTPUT_FILE}
+      BYPRODUCTS ${ASM_OUTPUT_FILE})
+  add_dependencies(copy_${name} ${name})
+  if (NOT ARG_CHECK_PREFIXES)
+    set(ARG_CHECK_PREFIXES "CHECK")
+  endif()
+  foreach(prefix ${ARG_CHECK_PREFIXES})
+    add_test(NAME run_${name}_${prefix}
+        COMMAND
+          ${LLVM_FILECHECK_EXE} ${name}.cc
+          --input-file=${ASM_OUTPUT_FILE}
+          --check-prefixes=CHECK,CHECK-${ASM_TEST_COMPILER}
+        WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
+  endforeach()
+endmacro()
+
diff --git a/test/BUILD b/test/BUILD
index c268221..dfca7db 100644
--- a/test/BUILD
+++ b/test/BUILD
@@ -42,4 +42,7 @@ cc_library(
     ] + (
         ["@com_google_googletest//:gtest_main"] if (test_src in NEEDS_GTEST_MAIN) else []
     ),
-) for test_src in glob(["*_test.cc"])]
+# FIXME: Add support for assembly tests to bazel.
+# See Issue #556
+# https://github.com/google/benchmark/issues/556
+) for test_src in glob(["*_test.cc"], exclude = ["*_assembly_test.cc"])]
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index ad1bd93..7c6366f 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -22,6 +22,12 @@ if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
   endforeach()
 endif()
 
+check_cxx_compiler_flag(-O3 BENCHMARK_HAS_O3_FLAG)
+set(BENCHMARK_O3_FLAG "")
+if (BENCHMARK_HAS_O3_FLAG)
+  set(BENCHMARK_O3_FLAG "-O3")
+endif()
+
 # NOTE: These flags must be added after find_package(Threads REQUIRED) otherwise
 # they will break the configuration check.
 if (DEFINED BENCHMARK_CXX_LINKER_FLAGS)
@@ -159,6 +165,25 @@ if (BENCHMARK_ENABLE_GTEST_TESTS)
   add_gtest(statistics_test)
 endif(BENCHMARK_ENABLE_GTEST_TESTS)
 
+###############################################################################
+# Assembly Unit Tests
+###############################################################################
+
+if (BENCHMARK_ENABLE_ASSEMBLY_TESTS)
+  if (NOT LLVM_FILECHECK_EXE)
+    message(FATAL_ERROR "LLVM FileCheck is required when including this file")
+  endif()
+  include(AssemblyTests.cmake)
+  add_filecheck_test(donotoptimize_assembly_test)
+  add_filecheck_test(state_assembly_test)
+  add_filecheck_test(clobber_memory_assembly_test)
+endif()
+
+
+
+###############################################################################
+# Code Coverage Configuration
+###############################################################################
 
 # Add the coverage command(s)
 if(CMAKE_BUILD_TYPE)
diff --git a/test/clobber_memory_assembly_test.cc b/test/clobber_memory_assembly_test.cc
new file mode 100644
index 0000000..f41911a
--- /dev/null
+++ b/test/clobber_memory_assembly_test.cc
@@ -0,0 +1,64 @@
+#include <benchmark/benchmark.h>
+
+#ifdef __clang__
+#pragma clang diagnostic ignored "-Wreturn-type"
+#endif
+
+extern "C" {
+
+extern int ExternInt;
+extern int ExternInt2;
+extern int ExternInt3;
+
+}
+
+// CHECK-LABEL: test_basic:
+extern "C" void test_basic() {
+  int x;
+  benchmark::DoNotOptimize(&x);
+  x = 101;
+  benchmark::ClobberMemory();
+  // CHECK: leaq [[DEST:[^,]+]], %rax
+  // CHECK: movl $101, [[DEST]]
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_redundant_store:
+extern "C" void test_redundant_store() {
+  ExternInt = 3;
+  benchmark::ClobberMemory();
+  ExternInt = 51;
+  // CHECK-DAG: ExternInt
+  // CHECK-DAG: movl $3
+  // CHECK: movl $51
+}
+
+// CHECK-LABEL: test_redundant_read:
+extern "C" void test_redundant_read() {
+  int x;
+  benchmark::DoNotOptimize(&x);
+  x = ExternInt;
+  benchmark::ClobberMemory();
+  x = ExternInt2;
+  // CHECK: leaq [[DEST:[^,]+]], %rax
+  // CHECK: ExternInt(%rip)
+  // CHECK: movl %eax, [[DEST]]
+  // CHECK-NOT: ExternInt2
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_redundant_read2:
+extern "C" void test_redundant_read2() {
+  int x;
+  benchmark::DoNotOptimize(&x);
+  x = ExternInt;
+  benchmark::ClobberMemory();
+  x = ExternInt2;
+  benchmark::ClobberMemory();
+  // CHECK: leaq [[DEST:[^,]+]], %rax
+  // CHECK: ExternInt(%rip)
+  // CHECK: movl %eax, [[DEST]]
+  // CHECK: ExternInt2(%rip)
+  // CHECK: movl %eax, [[DEST]]
+  // CHECK: ret
+}
diff --git a/test/donotoptimize_assembly_test.cc b/test/donotoptimize_assembly_test.cc
new file mode 100644
index 0000000..d4b0bab
--- /dev/null
+++ b/test/donotoptimize_assembly_test.cc
@@ -0,0 +1,163 @@
+#include <benchmark/benchmark.h>
+
+#ifdef __clang__
+#pragma clang diagnostic ignored "-Wreturn-type"
+#endif
+
+extern "C" {
+
+extern int ExternInt;
+extern int ExternInt2;
+extern int ExternInt3;
+
+inline int Add42(int x) { return x + 42; }
+
+struct NotTriviallyCopyable {
+  NotTriviallyCopyable();
+  explicit NotTriviallyCopyable(int x) : value(x) {}
+  NotTriviallyCopyable(NotTriviallyCopyable const&);
+  int value;
+};
+
+struct Large {
+  int value;
+  int data[2];
+};
+
+}
+// CHECK-LABEL: test_with_rvalue:
+extern "C" void test_with_rvalue() {
+  benchmark::DoNotOptimize(Add42(0));
+  // CHECK: movl $42, %eax
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_with_large_rvalue:
+extern "C" void test_with_large_rvalue() {
+  benchmark::DoNotOptimize(Large{ExternInt, {ExternInt, ExternInt}});
+  // CHECK: ExternInt(%rip)
+  // CHECK: movl %eax, -{{[0-9]+}}(%[[REG:[a-z]+]]
+  // CHECK: movl %eax, -{{[0-9]+}}(%[[REG]])
+  // CHECK: movl %eax, -{{[0-9]+}}(%[[REG]])
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_with_non_trivial_rvalue:
+extern "C" void test_with_non_trivial_rvalue() {
+  benchmark::DoNotOptimize(NotTriviallyCopyable(ExternInt));
+  // CHECK: mov{{l|q}} ExternInt(%rip)
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_with_lvalue:
+extern "C" void test_with_lvalue() {
+  int x = 101;
+  benchmark::DoNotOptimize(x);
+  // CHECK-GNU: movl $101, %eax
+  // CHECK-CLANG: movl $101, -{{[0-9]+}}(%[[REG:[a-z]+]])
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_with_large_lvalue:
+extern "C" void test_with_large_lvalue() {
+  Large L{ExternInt, {ExternInt, ExternInt}};
+  benchmark::DoNotOptimize(L);
+  // CHECK: ExternInt(%rip)
+  // CHECK: movl %eax, -{{[0-9]+}}(%[[REG:[a-z]+]])
+  // CHECK: movl %eax, -{{[0-9]+}}(%[[REG]])
+  // CHECK: movl %eax, -{{[0-9]+}}(%[[REG]])
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_with_non_trivial_lvalue:
+extern "C" void test_with_non_trivial_lvalue() {
+  NotTriviallyCopyable NTC(ExternInt);
+  benchmark::DoNotOptimize(NTC);
+  // CHECK: ExternInt(%rip)
+  // CHECK: movl %eax, -{{[0-9]+}}(%[[REG:[a-z]+]])
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_with_const_lvalue:
+extern "C" void test_with_const_lvalue() {
+  const int x = 123;
+  benchmark::DoNotOptimize(x);
+  // CHECK: movl $123, %eax
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_with_large_const_lvalue:
+extern "C" void test_with_large_const_lvalue() {
+  const Large L{ExternInt, {ExternInt, ExternInt}};
+  benchmark::DoNotOptimize(L);
+  // CHECK: ExternInt(%rip)
+  // CHECK: movl %eax, -{{[0-9]+}}(%[[REG:[a-z]+]])
+  // CHECK: movl %eax, -{{[0-9]+}}(%[[REG]])
+  // CHECK: movl %eax, -{{[0-9]+}}(%[[REG]])
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_with_non_trivial_const_lvalue:
+extern "C" void test_with_non_trivial_const_lvalue() {
+  const NotTriviallyCopyable Obj(ExternInt);
+  benchmark::DoNotOptimize(Obj);
+  // CHECK: mov{{q|l}} ExternInt(%rip)
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_div_by_two:
+extern "C" int test_div_by_two(int input) {
+  int divisor = 2;
+  benchmark::DoNotOptimize(divisor);
+  return input / divisor;
+  // CHECK: movl $2, [[DEST:.*]]
+  // CHECK: idivl [[DEST]]
+  // CHECK: ret
+}
+
+// CHECK-LABEL: test_inc_integer:
+extern "C" int test_inc_integer() {
+  int x = 0;
+  for (int i=0; i < 5; ++i)
+    benchmark::DoNotOptimize(++x);
+  // CHECK: movl $1, [[DEST:.*]]
+  // CHECK: {{(addl \$1,|incl)}} [[DEST]]
+  // CHECK: {{(addl \$1,|incl)}} [[DEST]]
+  // CHECK: {{(addl \$1,|incl)}} [[DEST]]
+  // CHECK: {{(addl \$1,|incl)}} [[DEST]]
+  // CHECK-CLANG: movl [[DEST]], %eax
+  // CHECK: ret
+  return x;
+}
+
+// CHECK-LABEL: test_pointer_rvalue
+extern "C" void test_pointer_rvalue() {
+  // CHECK: movl $42, [[DEST:.*]]
+  // CHECK: leaq [[DEST]], %rax
+  // CHECK-CLANG: movq %rax, -{{[0-9]+}}(%[[REG:[a-z]+]])
+  // CHECK: ret
+  int x = 42;
+  benchmark::DoNotOptimize(&x);
+}
+
+// CHECK-LABEL: test_pointer_const_lvalue:
+extern "C" void test_pointer_const_lvalue() {
+  // CHECK: movl $42, [[DEST:.*]]
+  // CHECK: leaq [[DEST]], %rax
+  // CHECK-CLANG: movq %rax, -{{[0-9]+}}(%[[REG:[a-z]+]])
+  // CHECK: ret
+  int x = 42;
+  int * const xp = &x;
+  benchmark::DoNotOptimize(xp);
+}
+
+// CHECK-LABEL: test_pointer_lvalue:
+extern "C" void test_pointer_lvalue() {
+  // CHECK: movl $42, [[DEST:.*]]
+  // CHECK: leaq [[DEST]], %rax
+  // CHECK-CLANG: movq %rax, -{{[0-9]+}}(%[[REG:[a-z+]+]])
+  // CHECK: ret
+  int x = 42;
+  int *xp = &x;
+  benchmark::DoNotOptimize(xp);
+}
diff --git a/test/state_assembly_test.cc b/test/state_assembly_test.cc
new file mode 100644
index 0000000..e2c5c86
--- /dev/null
+++ b/test/state_assembly_test.cc
@@ -0,0 +1,66 @@
+#include <benchmark/benchmark.h>
+
+#ifdef __clang__
+#pragma clang diagnostic ignored "-Wreturn-type"
+#endif
+
+extern "C" {
+  extern int ExternInt;
+  benchmark::State& GetState();
+  void Fn();
+}
+
+using benchmark::State;
+
+// CHECK-LABEL: test_for_auto_loop:
+extern "C" int test_for_auto_loop() {
+  State& S = GetState();
+  int x = 42;
+  // CHECK: 	[[CALL:call(q)*]]	_ZN9benchmark5State16StartKeepRunningEv
+  // CHECK-NEXT: testq %rbx, %rbx
+  // CHECK-NEXT: je [[LOOP_END:.*]]
+
+  for (auto _ : S) {
+    // CHECK: .L[[LOOP_HEAD:[a-zA-Z0-9_]+]]:
+    // CHECK-GNU-NEXT: subq $1, %rbx
+    // CHECK-CLANG-NEXT: {{(addq \$1,|incq)}} %rax
+    // CHECK-NEXT: jne .L[[LOOP_HEAD]]
+    benchmark::DoNotOptimize(x);
+  }
+  // CHECK: [[LOOP_END]]:
+  // CHECK: [[CALL]]	_ZN9benchmark5State17FinishKeepRunningEv
+
+  // CHECK: movl $101, %eax
+  // CHECK: ret
+  return 101;
+}
+
+// CHECK-LABEL: test_while_loop:
+extern "C" int test_while_loop() {
+  State& S = GetState();
+  int x = 42;
+
+  // CHECK: j{{(e|mp)}} .L[[LOOP_HEADER:[a-zA-Z0-9_]+]]
+  // CHECK-NEXT: .L[[LOOP_BODY:[a-zA-Z0-9_]+]]:
+  while (S.KeepRunning()) {
+    // CHECK-GNU-NEXT: subq $1, %[[IREG:[a-z]+]]
+    // CHECK-CLANG-NEXT: {{(addq \$-1,|decq)}} %[[IREG:[a-z]+]]
+    // CHECK: movq %[[IREG]], [[DEST:.*]]
+    benchmark::DoNotOptimize(x);
+  }
+  // CHECK-DAG: movq [[DEST]], %[[IREG]]
+  // CHECK-DAG: testq %[[IREG]], %[[IREG]]
+  // CHECK-DAG: jne .L[[LOOP_BODY]]
+  // CHECK-DAG: .L[[LOOP_HEADER]]:
+
+  // CHECK: cmpb $0
+  // CHECK-NEXT: jne .L[[LOOP_END:[a-zA-Z0-9_]+]]
+  // CHECK: [[CALL:call(q)*]] _ZN9benchmark5State16StartKeepRunningEv
+
+  // CHECK: .L[[LOOP_END]]:
+  // CHECK: [[CALL]] _ZN9benchmark5State17FinishKeepRunningEv
+
+  // CHECK: movl $101, %eax
+  // CHECK: ret
+  return 101;
+}
diff --git a/tools/strip_asm.py b/tools/strip_asm.py
new file mode 100755
index 0000000..9030550
--- /dev/null
+++ b/tools/strip_asm.py
@@ -0,0 +1,151 @@
+#!/usr/bin/env python
+
+"""
+strip_asm.py - Cleanup ASM output for the specified file
+"""
+
+from argparse import ArgumentParser
+import sys
+import os
+import re
+
+def find_used_labels(asm):
+    found = set()
+    label_re = re.compile("\s*j[a-z]+\s+\.L([a-zA-Z0-9][a-zA-Z0-9_]*)")
+    for l in asm.splitlines():
+        m = label_re.match(l)
+        if m:
+            found.add('.L%s' % m.group(1))
+    return found
+
+
+def normalize_labels(asm):
+    decls = set()
+    label_decl = re.compile("^[.]{0,1}L([a-zA-Z0-9][a-zA-Z0-9_]*)(?=:)")
+    for l in asm.splitlines():
+        m = label_decl.match(l)
+        if m:
+            decls.add(m.group(0))
+    if len(decls) == 0:
+        return asm
+    needs_dot = next(iter(decls))[0] != '.'
+    if not needs_dot:
+        return asm
+    for ld in decls:
+        asm = re.sub("(^|\s+)" + ld + "(?=:|\s)", '\\1.' + ld, asm)
+    return asm
+
+
+def transform_labels(asm):
+    asm = normalize_labels(asm)
+    used_decls = find_used_labels(asm)
+    new_asm = ''
+    label_decl = re.compile("^\.L([a-zA-Z0-9][a-zA-Z0-9_]*)(?=:)")
+    for l in asm.splitlines():
+        m = label_decl.match(l)
+        if not m or m.group(0) in used_decls:
+            new_asm += l
+            new_asm += '\n'
+    return new_asm
+
+
+def is_identifier(tk):
+    if len(tk) == 0:
+        return False
+    first = tk[0]
+    if not first.isalpha() and first != '_':
+        return False
+    for i in range(1, len(tk)):
+        c = tk[i]
+        if not c.isalnum() and c != '_':
+            return False
+    return True
+
+def process_identifiers(l):
+    """
+    process_identifiers - process all identifiers and modify them to have
+    consistent names across all platforms; specifically across ELF and MachO.
+    For example, MachO inserts an additional understore at the beginning of
+    names. This function removes that.
+    """
+    parts = re.split(r'([a-zA-Z0-9_]+)', l)
+    new_line = ''
+    for tk in parts:
+        if is_identifier(tk):
+            if tk.startswith('__Z'):
+                tk = tk[1:]
+            elif tk.startswith('_') and len(tk) > 1 and \
+                    tk[1].isalpha() and tk[1] != 'Z':
+                tk = tk[1:]
+        new_line += tk
+    return new_line
+
+
+def process_asm(asm):
+    """
+    Strip the ASM of unwanted directives and lines
+    """
+    new_contents = ''
+    asm = transform_labels(asm)
+
+    # TODO: Add more things we want to remove
+    discard_regexes = [
+        re.compile("\s+\..*$"), # directive
+        re.compile("\s*#(NO_APP|APP)$"), #inline ASM
+        re.compile("\s*#.*$"), # comment line
+        re.compile("\s*\.globa?l\s*([.a-zA-Z_][a-zA-Z0-9$_.]*)"), #global directive
+        re.compile("\s*\.(string|asciz|ascii|[1248]?byte|short|word|long|quad|value|zero)"),
+    ]
+    keep_regexes = [
+
+    ]
+    fn_label_def = re.compile("^[a-zA-Z_][a-zA-Z0-9_.]*:")
+    for l in asm.splitlines():
+        # Remove Mach-O attribute
+        l = l.replace('@GOTPCREL', '')
+        add_line = True
+        for reg in discard_regexes:
+            if reg.match(l) is not None:
+                add_line = False
+                break
+        for reg in keep_regexes:
+            if reg.match(l) is not None:
+                add_line = True
+                break
+        if add_line:
+            if fn_label_def.match(l) and len(new_contents) != 0:
+                new_contents += '\n'
+            l = process_identifiers(l)
+            new_contents += l
+            new_contents += '\n'
+    return new_contents
+
+def main():
+    parser = ArgumentParser(
+        description='generate a stripped assembly file')
+    parser.add_argument(
+        'input', metavar='input', type=str, nargs=1,
+        help='An input assembly file')
+    parser.add_argument(
+        'out', metavar='output', type=str, nargs=1,
+        help='The output file')
+    args, unknown_args = parser.parse_known_args()
+    input = args.input[0]
+    output = args.out[0]
+    if not os.path.isfile(input):
+        print(("ERROR: input file '%s' does not exist") % input)
+        sys.exit(1)
+    contents = None
+    with open(input, 'r') as f:
+        contents = f.read()
+    new_contents = process_asm(contents)
+    with open(output, 'w') as f:
+        f.write(new_contents)
+
+
+if __name__ == '__main__':
+    main()
+
+# vim: tabstop=4 expandtab shiftwidth=4 softtabstop=4
+# kate: tab-width: 4; replace-tabs on; indent-width 4; tab-indents: off;
+# kate: indent-mode python; remove-trailing-spaces modified;